Bangkok's tall buildings sitting on soft, shaky ground

Shawn Kelly
Special to The Nation May 21, 2014 1:00 am

Quake damage in Chiang Rai.

The capital is vulnerable to distant large earthquakes, new BMA-commissioned study finds

Not many saw it coming. The strongest ever earthquake with an epicentre in Thailand caught the country off guard as it struck the North with deadly intensity. Killing one and injuring twenty-three, the powerful 6.3-magnitude shudder that shook Chiang Rai province on May 5 cleaved gaping fissures in highways and damaged thousands of homes, Buddhist temples, ancient heritage sites, schools, hospitals and utilities.

The quake frightened an unsuspecting populace who scurried for safety in the early evening twilight - and even rattled the nerves of people inside swaying skyscrapers in distant Bangkok. Still, it was not the first time the capital's terra firma has moved.

When the deadly Great Sichuan Earthquake killed 70,000 Chinese in 2008, its powerful shockwaves rippled south to Thailand 2,000 kilometres away. And when a magnitude-6.8 tremor killed 13 people in the Mandalay region of Myanmar on November 11, 2012, it too was felt over 700km away in the nation's metropolis.

To be sure, the country is not as exposed to earthquakes as say Japan, Indonesia or New Zealand, and it does not sit directly atop the fault lines of the Pacific Ring of Fire, where 90 per cent of the world's earthquakes occur. However, as the Chiang Rai upheaval proved, the Kingdom is far from immune to their terror.

Witness the immense loss of life and catastrophic damage from the tsunami unleashed by the giant quake of December 2004 off the coast of Sumatra, and one can see just how dangerous the country's neighbourhood can be. Add to the volatile mix the 14 known fault lines that criss-cross the nation's North and Western border frontier - including the newly discovered 30km Ongkarak fault passing through Nakhon Nayok, Saraburi and Lop Buri provinces - and Greater Bangkok's 10 million-plus residents may be vulnerable to seismic hazards.

In fact, distant large earthquakes occurring over the horizon are a genuine threat to Bangkok. That's the sobering warning Dr Pennung Warnitchai, a noted seismological expert at the Asian Institute of Technology (AIT), has been sounding for some time. The structural engineer said that when an extremely large quake eventually rumbles across the region many structures in the capital will suffer serious damage and some with structural weaknesses will collapse.

Now, researchers from AIT, Thammasat and Mahidol universities have the smoking gun to back up that claim. As part of a one-year research project completed for the Disaster Prevention and Mitigation Department of the Bangkok Metropolitan Administration (BMA), three teams of experts led by Pennung produced a statistical model that identifies the impact to the city's skyscrapers, including 1,434 buildings taller than 12 storeys and 645 above 20 floors.

Specialised 3-D software simulates how the buildings fare under severe ground-motion scenarios. After crunching the data, scientists concluded that some four to 17 buildings of between 12 and 88 storeys - or 1 per cent of the buildings in Bangkok - are expected to collapse during a distant large quake.

Indeed, the proliferation of older high-rises built on top of Bangkok's uniquely soft soil basin could mean big trouble, and Pennung points to the Michoacan Earthquake that devastated Mexico City in 1985 as a frightening example of what could happen here. That 8.1-magnitude temblor occurred in the Pacific Ocean off the coast of the Mexican state of Michoacan and was 350-400km away from Mexico's capital, but still killed over 10,000 people when building-after-building in the city collapsed.

Experts explain that when energy waves released by an earthquake pass through the earth, they are filtered in different ways by various kinds of soils. Hard bedrock can absorb more earthquake energy than sandy soils or landfill, so buildings built on solid rock are more resilient to shaking than those rooted in softer earth. Thick soft surficial deposits can amplify earthquake ground motions considerably.

In the aftermath of the Mexico City disaster, seismologists discovered that the soft mud of a former lakebed under the city actually boosted the tremor's vibrations, despite its seemingly safe distance from the epicentre. Seismic shock waves quietly crested below the surface and randomly matched some structures' natural sway or resonant frequencies. That multiplied the energy passing through the buildings and greatly amplified the shaking effect, causing hundreds to topple. As Mexico City tragically illustrated, the characteristics of an earthquake and the ground below a building can coincide to create a perfect storm for disaster.

Bangkok is now faced with a very similar set of circumstances. Subterranean energy waves generated by a massive quake with its epicentre far from the city in the Andaman Sea or in Myanmar could reach the soft earthen basin and jolt the population like never before. "We have records to prove that Bangkok itself will shake three to four times more than adjacent areas outside the basin," warns Pennung.

Particularly worrisome is the Sagaing Fault Line and the Indian Ocean Subduction Zone, each situated to the west of Thailand. The Sagaing Fault in east-central Myanmar is just 400km from Bangkok and is capable of generating a 7.5 to 8.0-magnitude quake. Moreover, the tectonic Subduction Zone located 600-700km west of the capital in the Andaman Sea is home to the Burma Plate that generated the 2004 Indian Ocean 9.3-magnitude mega-thrust earthquake, which was the third-strongest in recorded history. Though a healthy 1,200km away, that quake shook Bangkok more than any other in recent times.

Casting a scientific gaze at the heavily populated capital region which includes Nonthaburi, Pathum Thani and Samut Prakan provinces, the engineers found that the basin's geology primarily comprises a layer of soft surface clay sitting above harder soils below. That's problematic because soft soils can trap the subterranean energy waves released by an earthquake, thus amplifying the ground's shaking at the surface. In the study, cutting-edge micro-tremor array tests set up at 50 locations in the capital measured the natural vibration levels of city soils. Crucially, the new knowledge has armed Thai scientists with the data they need to estimate the amplification of the ground during a big quake. "It's like what happens when you shake a bowl filled with gelatin - the surface moves more than the bottom," Pennung explains.

Analysing earthquake statistics from the last 90 years - which include data on size, location, time and epicentre depth of all seismic activity - researchers discovered that over 70 years ago a 7.3 level earthquake with an epicentre 600km from Thailand severely shook Bangkok. That was at a time when there were very few high-rises. Nowadays, however, Bangkok is one of the world's most vertical cities, ranking sixth after Hong Kong, New York City, Tokyo, Shanghai and Dubai with 355 skyscrapers taller than 100 metres. A scan across the cityscape reveals a vast concentration of structures 35 metres or higher.

After the Boxing Day 2004 tsunami catastrophe, national awareness of the dangers of earthquakes increased dramatically. In 2007, the capital was designated as earthquake-prone and legislation stipulated that new buildings 15 metres or higher must be built to withstand a 7.0-magnitude quake. Yet the law is not retroactive, and countless older high buildings constructed before that year still dot the land without any consideration of seismic loading.

Thailand was shocked to see a home-grown earthquake scar the land in the North. It arrived out of the blue, and was considered a wake-up call for action. Now a second alarm bell needs sounding - this time alerting a shaken country to the distant underground menace facing all-important Bangkok, home to canyons of skyscrapers situated on soft, shaky ground.